Solidify a complex hollow object in Blender
Solid is a relative term : ) especially in 3D computer graphics. Everything that looks solid in (almost) any 3D environment on a computer is made up out of points(or vertices) in space with lines(or edges) between them, with planes(or faces) filling up the space between the lines. But nothing visually fills up the space between these elements, and it is essentially a shell. Even though properties like volume and mass are used in physics simulation, this shell is (partly) used to derive values for such properties. So there is no need for &quot;solidifying the inside&quot;.
The Solidify modifier is a modeling tool and can be used to create such a shell around a mesh that might otherwise not be considered solid, like a plane with no thickness. The Solidify modifier can give a plane the appearance of a box which has thickness, and if it has no holes anywhere in its shell, is considered solid.